Comprehensive Income And Accumulated Other Comprehensive Loss - Reclassification out of AOCI (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating (expense) income, net	$ (4)	$ 10	$ 18	$ 38
Total before income taxes	619	599	2,239	1,829
Provision for income taxes	(145)	(132)	(452)	(366)
Net income attributable to Moody's	474	467	1,787	1,464
Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Net income attributable to Moody's	(3)	(2)	(13)	(5)
Losses on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Provision for income taxes	0	0	0	0
Net income attributable to Moody's	(1)	0	(2)	(1)
Losses on cash flow hedges | Reclassification out of Accumulated Other Comprehensive Income | Interest rate contract
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating (expense) income, net	(1)	0	(2)	(1)
Net Investment Hedges | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Provision for income taxes			(1)	0
Net income attributable to Moody's			1	0
Net Investment Hedges | Reclassification out of Accumulated Other Comprehensive Income | FX forwards
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating (expense) income, net			2	0
Total pension and other retirement benefits | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Total before income taxes	(3)	(2)	(16)	(5)
Provision for income taxes	1	0	4	1
Net income attributable to Moody's	(2)	(2)	(12)	(4)
Amortization of actuarial losses and prior service costs included in net income | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating (expense) income, net	(2)	(2)	(8)	(5)
Settlement charge | Reclassification out of Accumulated Other Comprehensive Income
Derivative Instruments, Gain (Loss) [Line Items]
Other non-operating (expense) income, net	$ (1)	$ 0	$ (8)	$ 0